The goal of this research is to evaluate the effects of soybean isoflavones on estrogen and isoflavone levels and action in premenopausal women. Eighteen subjects will consume 3 diets for about 13.5 weeks each: their habitual diets supplemented with an isoflavone-free soy powder (control diets), a low isoflavone powder diet and a high isoflavone powder diet. The soy product will be soy milk. A number of estrogen endpoints will be measured, reflecting estrogen availability, metabolism, excretion and action. Blood, urine, feces, food, exercise and menstrual diaries will be collected and endometrial biopsies will be performed.